The present invention relates to a fixing device for use in an image forming apparatus, which fixes a developer image formed on an object, thereby obtaining a fixed image.
In a fixing device built in a copying apparatus using an electrophotographic process, a developer image or a developer on a material subjected to a fixing process is heated and melted, and the developer is fixed on the material.
As methods of heating the developer, there are conventionally known a method of using a halogen lamp (a filament lamp utilizing resistor heating) and a method of using a flash light type lamp (a discharge lamp in which power is converted to heat energy). The method of using the flash light type lamp is not widely used. In addition to the method of converting power to heat energy with use of a lamp, there is known an induction heating method of producing heat by supplying a current to a magnetic field.
In the method of using the halogen lamp as heating source, such a structure is widely used that a tubular halogen lamp is disposed within at least one of a pair of rollers, which is formed in a hollow cylindrical shape, the rollers being capable of applying pressure to the material on which the developer is to be fixed and to the developer. In this structure, the roller in which the halogen lamp is disposed forms an operating portion (rotary contact portion) at a position contacting the other roller, and pressure and heat is applied to the material (and developer) guided to the rotary contact portion.
In many cases, the roller in which the halogen lamp is disposed is formed of a metal in order to maintain heat conduction. The other roller is formed of elastic material so that it may come into close contact with the metal roller at the rotary contact portion for contact with the metal roller.
When the material on which the developer is to be fixed passes by the rotary contact portion, the material receives heat mainly from the metallic roller. When the material is pressed on the metallic roller by the roller with elasticity, the thermally melted developer is captured and thus the developer is fixed on the material.
In the fixing device using the above roller, electrical energy is converted to light and heat and transmitted to the metallic roller by radiation. Subsequently, the outer periphery of the metallic roller is heated by conduction and predetermined heat is supplied to the material. Thus, the heat use efficiency is about 70%. Besides, since the roller is heated from inside, a great deal of time and power is required to raise the temperature of the outer periphery of the roller up to a temperature (e.g. 180.degree. C.) necessary for fixing the developer on the material. The surface temperature of the metallic roller is controlled at a target temperature by operating a switching element provided in a power supply device to turn on/off power voltage to the lamp, on the basis of the temperature detected by a thermistor or temperature sensing means.
However, in a case where the power supply voltage to the lamp is controlled by sensing the surface temperature of the metallic roller, even if the power is shut off at the time when the target temperature has just been reached, the temperature overshoots the target temperature due to the thickness of the roller. For example, there may occur such an offset phenomenon that a developer on the roller, which is applied in the first rotation of the roller, is transferred on the image on the roller which is applied in the second rotation of the roller.
In order to shorten the time needed to raise the surface temperature of the roller and to decrease a difference between the surface temperature and the target temperature, there is known a method of thinning the metallic roller and reducing the thermal capacity. On the other hand, when a thin roller is used, a local temperature variation occurs in the axial direction of the roller in association with the size of the material or recording sheet. Besides, when the number of recording sheets on which developer is to be fixed is plural and the sheets are successively fed to the rotary contact portion, the temperature of the outer periphery of the roller varies in association with the order of fed sheets and consequently a fixing ratio indicating the degree of attachment of developer on sheets varies.
As has been described above, the reduction in thickness of the metallic roller requires an intricate control for preventing a local temperature variation in the axial direction of the roller or a structure of the halogen lamp capable of providing different temperatures in the length direction of the roller. In addition, in order to prevent a variation in fixing ratio due to successive feeding of sheets, it is necessary to provide a margin to the maximum value of heat amount which can be produced by the halogen lamp or to provide a plurality of halogen lamps and vary the number of lamps to be turned on.
These measures inevitably lead to an increase in the cost of the fixing device (copying apparatus).
Examples of the fixing device using induction heating are described in Jpn. Pat. Appln. KOKAI Publication No. 8-16005 and Jpn. Pat. Appln. KOKAI Publication No. 8-44227.
The induction heating device includes a coil for supplying electric current to produce a predetermined magnetic field and a core bent toward the rotary contact portion in order to collect at the rotary contact portion the magnetic field produced by the current supplied to the coil. There is a problem, however, in that the magnetic field produced by the current supplied to the coil is symmetric with respect to an axis connecting the center of the core and the center of the rotary contact portion and thus the center of generated heat distribution does not coincide with the center of the rotary contact portion and the generated heat distribution exhibits two peaks on the upstream and downstream sides in the direction in which the material on which developer is fixed is fed, and not at the center of the rotary contact portion. In this case, the amount of heat that can be used at the rotary contact portion decreases and as a result the efficiency of use of current (electric energy) supplied to the coil deteriorates.
Furthermore, since the coil is disposed on a side of the core closer to the rotary contact portion and on the opposite side of the core, with the core sandwiched by the coil, a region farthest from the rotary contact portion between the elastic roller and the belt is heated undesirably.